Abstract
Polylactide (PLA) is an environmentally friendly polymer derived from biomasses, and has been emerged as an alternative to conventional petroleum-based polymeric materials. Although PLA is a high-strength and high-modulus polymer analogous to PET, its inherent brittleness and low toughness restrict the range of applications. The main purpose of this research is to modify the mechanical properties of PLA and enhance the toughness of PLA. This thesis comprises three sections.In the first section, a thermoplastic polyole?n elastomer-graft- polylactide (TPO-g-PLA) was prepared by grafting polylactide onto maleic anhydride-functionalized TPO (TPO-g-MAH) in the presence of 4-dimethylaminopyridine (4-DMAP). The structures of the TPO-g-PLA copolymers were conducted by FT-IR and 1H-NMR. The effects of reaction temperature and concentration of 4-DMAP on the reactivity of graft polymerization were investigated by FT-IR, which revealed that a high reaction temperature and a high DMAP concentration are associated with dramatic depolymerization of PLA and reduction of steric hindrance effect in the graft reaction. Upon addition of the graft-type copolymers, acting as a premade compatibilizer, the compatibility of the PLA/TPO blend system was significantly improved. As the concentration of TPO-g-PLA copolymer increased, the tensile toughness and elongation at break increased with compatibilizer concentration up to 2.5 wt%, beyond which it declined. The effect of the chemical compositions of the TPO-g-PLA copolymers on the efficiency of compatibilization and mechanical properties of the PLA/TPO blends was examined by altering the number of grafting sites and concentration of 4-DMAP, suggesting that DMAP concentration dominated the properties of the ternary blend materials. Two compatibilizers, TPO-g-MAH and TPO-g-PLA, were used to compatibilize the PLA/TPO blends; the results suggested that TPO-g-PLA was more efficient in reducing the interfacial tension between the two immiscible polymers and in improving the mechanical properties of PLA/TPO blending specimens.In the second section, the (AB)n-type multiblock copolymers containing poly(L-lactide) (PLLA) and poly(dimethyl siloxane) (PDMS) segments were synthesized by chain extension of the hydroxyl-telechelic PLLA-PDMS-PLLA triblock copolymers, which were prepared by the ring-opening polymerization of L-lactide initiated by α,ω-functionalized hydroxyl poly(dimethyl siloxane), using 1,6-hexamethylene diisocyanate as a chain extender. The triblock and the multiblock copolymers were characterized by FT-IR, 1H-NMR and GPC. The effect of the chemical composition of the triblock copolymers, including the molecular weight and the constitutive segment chain length of the macrodiol, on the development of the Mw of the multiblock was discussed based on diffusion effect. Furthermore, the consumption of the isocyanate groups was determined by FT-IR to investigate the dependence of the reaction kinetics of the urethane formation on the chemical composition of the triblock copolymer. The results reveal that the order of the chain extension reaction depended on the Mw of the triblock copolymer: a second order reaction was transformed into a third reaction as the Mw of the triblock copolymer increased from 7000 to 25,000 (g/mol) perhaps because of the inhibition of the formation of an active complex involved in the catalyzed-urethane reaction by the polymer chain aggregation. Finally, the mechanical properties of the multiblock copolymers demonstrated that the introduction of the extremely ?exible PDMS segment substantially improved the elongation at breakage, and the tensile strength and the tensile modulus declined due to the intrinsic elasticity of such segments.In the third section, the behaviors of microphase separation and the crystallization kinetics for triblock copolymers composed of PLLA and PDMS were characterized. From the results of thermal analysis, two glass transition temperatures which were measured by DSC showed the occurrence of phase separation phenomena in the PLLA-PDMS-PLLA triblock copolymers. And their the molten morphologies following isothermal crystallization of were characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The break-out and preservation of the nanostructure of the triblock copolymer depended on the segregation strength, which was manipulated by varying the degree of polymerization. The crystallization kinetics of these semicrystalline copolymers and the effect of isothermal crystallization on their melting behaviors were also studied using DSC, FT-IR and WAXS. The exclusive presence of α-phase PLLA crystallite was verified by identifying the absence of the WAXS diffraction signal at 2θ = 24.5° and the presence of IR absorption at 1749 cm-1 when the PLLA segment of the block copolymers was present as a minor component. The dependence of the crystallization rate (Rc) on the chemical composition of the triblock copolymers reveals that the Rc of the triblock copolymers was lower than that of PLLA homopolymer and the Rc were substantially reduced when the minor component of the crystallizable PLLA domains was dispersed in the PDMS matrix.